The present invention relates to substances capable of forming chemical crosslinks between macromolecules of polymers, and more particularly to a crosslinking agent, to a method of producing same and to paper made with the use thereof.
The present invention can be used in various branches of industry, since it refers to the field of double-stage processes for fabricating articles from polymers.
The double-stage nature of these processes involves a first stage in which an article is shaped from a solution or melt of a polymer capable of dissolving or melting, while during the second stage, the polymer, in the form of a shaped article, is rendered nonmelting and insoluble by chemically crosslinking its macromolecules. Crosslinking can be carried out through radiation, thermal, or chemical treatment. In the latter case special bi- and polyfunctional chemical compounds, known as crosslinking agents are used, and the physico-chemistry of the double-stage character of these processed polymers does not necessarily rule out the possibility their realization in one technological stage in a single apparatus or unit.
For crosslinking a wide range of polymers various crosslinking agents are used, depending upon the kind and number of groups in the polymer prone to chemical crosslinking.
Crosslinking of hydroxyl groups of polymers in an aqueous solution presents greater difficulties than crosslinking of carboxyl, amine, imino or amide groups of water-soluble polymers. Organic and inorganic substances and mixtures thereof are used as crosslinking agents for hydroxyl-containing water-soluble polymers.
Producing diverse polymeric coatings is a particular field where a double-stage process may be used. Ease, safety and technological efficiency of producing is required of the coating production techniques, whereas resistance to outside effects, such as moisture and atmospheric oxygen, resistance to various active chemicals, solvents and temperature gradients over a long period of time is required of the coating applied. The coatings formed from chemically crosslinked hydroxyl-containing water-soluble polymers by treating said polymers with crosslinking agents particularly meet all the above-mentioned requirements.
Fabricating nonwoven materials requiring formation of the links between the fibers of the nonwoven material base, resistant to atmospheric effects, secretions of the human body, foodstuffs and laundering is another field utilizing double-stage processes. Such resistance can be achieved by impregnating the fibers with hydroxyl-containing water-soluble polymers, followed by crosslinking of the latter.
One of the applications of double-stage processes is bonding together various surfaces with crosslinking adhesives, whereby the bonding effect can be achieved through the use of hydroxyl-containing water-soluble polymers crosslinked after the adhesive layer has dried up. A very important requirement is fulfilled therewith: crosslinking can proceed with sufficient rapidity at room temperature of 25.degree. .+-. 10.degree. C.
Production of man-made fibers which can be spun from aqueous solutions of hydroxyl-containing polymers and crosslinked after their spinning is a special field of realization of double-stage processes. These fibers are highly hydrophilic while clothing made from such fibers is hygienic wearwise and possesses low-crease properties.
It is also expedient to employ a double-stage process for sizing textile fibers, using solutions of hydroxyl-containing water-soluble polymers, as sizing media, such polymers, being chemically crosslinked after application on the fibers. Such sizing media can be easily imparted with antistatic, friction, hydrophilic and delustering properties.
The double-stage process can also be used for the fabrication of sound- and heat-insulation boards with the use of aqueous solutions of crosslinked hydroxyl-containing polymers as binders for fibrous materials. A field of traditionally wide use of crosslinked coatings produced through a double-stage process from hydroxyl-containing water-soluble polymers in papermaking, where these coatings are applied using size presses, calenders, paper coating and baryta coating machines and ensure high resistance to water penetration combined with high hydrophilic capacity of the surface. These coatings can be used for making writing, printing and packing paper, as well as for making a number of technical grades of paper such as base of photographic, drafting, drawing, diagram and cartographic paper, paper for office-keeping and paper for computing machinery.
A new field of application for a double-stage process is production of synthetic paper, particularly of synthetic film paper where the crosslinked film itself, formed from an aqueous solution of hydroxyl-containing polymers, possesses paper-like properties, and can be used in a number of specific cases where "conventional" paper is inapplicable.
On the one hand, most diverse bi- and polyfunctional substances, both organic and inorganic, can be used as crosslinking agents, since only the capability of forming chemical crosslinks between at least two molecules of the polymer is required of the crosslinking agent molecule. On the other hand, specific classes of crosslinking agents are required for each type of the polymer functional groups. A large number of water soluble film-forming polymers contain hydroxyl groups. Some very important water-soluble polymers, e.g. polyvinyl alcohol or starch, do not contain any other groups prone to chemical crosslinking except hydroxyl groups.
Crosslinking of hydroxyl groups in an aqueous solution is a very complicated task, since the water molecule is a hydroxyl bonded with hydrogen. Therefore, the majority of compounds capable of reacting with the hydroxyl group resulting in the formation of chemical crosslinks, will actively interact, first of all, with the water molecules forming chemical compounds incapable of further interaction with the polymer hydroxyl groups. Since the concentration of aqueous solutions of polymers does not normally exceed 20 wt.% and their molecular weight is not below 2,000, molar water concentration (content: 80 wt.%, molecular weight: 18) is hundreds of times higher than the polymer molar concentration (in the above-mentioned extreme case 20% polymer with the molecular weight of 2,000, by a factor of 440). Hence, it is essential that crosslinking agents be used which interact selectively only with the polymer hydroxyls and do not react with hydroxyls of water non-reactive products. For this reason the majority of crosslinking agents are inapplicable for crosslinking hydroxyl groups in aqueous solutions of polymers. Here it will be appropriate to explain why particular attention is paid to water-soluble hydroxyl-containing polymers.
In order to apply an integral non-porous coating film e.g. on paper, solutions of film-forming materials with maximum possible concentrations should be used, since in case of maximum concentration of the solution, the reduction in the layer volume during drying will be minimum. This statement is based on the fact that the polymer solution is transformed to a non-flowing state when the polymer concentration in the solution equals to 1-25 wt.%. After the polymer solution has been transformed to a non-flowing state, gel for example, the initial volume of gel under fast drying conditions decreases nonlinearly in relation to the amount of solvent being removed, namely, the solvent removal rate is ahead of the polymeric gel volume decrease, due to the fact that the stress relaxation time in the polymeric gel is comparable with the solvent removal rate and increases as the solvent is being removed. Therefore, the polymeric carcass of the gel undergoes evergrowing stresses during drying, and its integrity is disturbed. The film of the resulting coating becomes porous, and is no longer a barrier to the penetration of liquid.
In addition, with higher concentrations of the solutions their viscosity increases, thus decreasing the solution penetration into paper and also promoting formation of an integral film on the paper surface. Many kinds of water-soluble polymers form fluid solutions with the polymer concentration over 10%.
For the majority of the paper grades a hydrophilic surface is required, i.e. the surface wetting angle with water should be below 90.degree.; application of a coating from water-soluble polymers is the simplest way of complying with said requirement. Hydrophilic properties, as has been already noted, are also required for nonwoven materials as well as for many kinds of textile fibers, and in these cases water-soluble polymers also offer easy solution of this problem.
The problem of the solvent removal always arises when using a polymer solution. Water is the only nontoxic and least expensive polymer solvent. When using aqueous solutions, the problems of sealing the equipment, of solvent recovery and fire hazards do not arise. Hence, it is always preferable to use aqueous solutions of polymers.
Aqueous dispersions of polymers also have the lastmentioned advantage of the presence of water as a dispersion medium, but dispersions have two substantial disadvantages as compared with polymer aqueous solutions.
The first disadvantage resides in that coalescence of the dispersion particles into an integral film occurs when heating to temperatures, as a rule, over 100.degree. C., which is difficult to attain during paper drying (due to its inevitable overdrying, unfavorably affecting all the paper properties) as well as during finishing textile fibers, synthetic fibers particularly.
The second specific disadvantage of aqueous dispersions of polymers is that they are liable to coagulation in the event of slight changes of external conditions and composition, for example, with changes in their temperature, pH, ionic strength of solution, or with addition of polyelectrolytes.
The above-mentioned factors are responsible for the important part played by water-soluble polymers and for the possibility of their extensive application in various branches of industry.
The overwhelming majority of the known film-forming water-soluble polymers contain hydroxyl groups. Hydroxylcontaining water-soluble polymers are proteins (gelatin, casein etc.), cellulose esters, starch and its derivatives, dextrines, acrylic and methacrylic acid copolymers, alginates and polyuronides (agar-agar, for example), polyvinyl alcohol and its water-soluble derivatives.
Hence, the agent capable of crosslinking hydroxyl groups of water-soluble polymers can find wide application. In most cases, for the reasons stated above, inorganic substances are used for chemical crosslinking of the hydroxyl-containing water-soluble polymers. Selective interaction of inorganic substances that are compounds of metals belonging to Groups II-VI and the iron Subgroup of the Periodic System is associated with the formation of chelate compounds.
Compounds of bivalent calcium, magnesium, zinc, copper and cobalt; trivalent boron, aluminum, chromium, iron and nickel; tetravalent tin, lead, titanium, zirconium and hafnium, as well as of pentavalent vanadium, taken either in combination or separately, are known in the art as inorganic crosslinking agents for hydroxyl-containing water-soluble polymers. Compounds of tetravalent titanium and zirconium, as well as of trivalent boron, taken either in combination or separately, are most extensively used as crosslinking agents for hydroxyl-containing water-soluble polymers.
If the criterion used for estimating the crosslinking activity of substances is the minimum ion concentration of a selected crosslinking agent, sufficient for the conversion of an aqueous polyvinyl alcohol solution with .gtoreq. 5 wt.% and .ltoreq. 15 wt.% of dry matter into a gel-like state, with the polymer being in the form of a crosslinked three-dimensional reticulum, within a specified period of time at a constant temperature, then compounds of tetravalent titanium display the highest crosslinking activity. All the other known inorganic crosslinking agents, including compounds of trivalent boron and tetravalent zirconium and hafnium, as well as their compounds with titanium, show much lower crosslinking activity. This is associated with the fact that in the titanium Subgroup of Group Iv of the Periodic System, titanium itself has the minimum ionic radius and, hence, maximum surface potential, governing maximum rate of chemical reactions. As compared to trivalent boron, tetravalent titanium has a higher charge and greater coordination number (4 for boron and 6 for titanium); therefore, titanium is a more active complexing agent than boron (chelate compounds are of intracomplex nature). Though the crosslinking activity of tetravalent titanium compounds is high, they are not the only inorganic crosslinking agents for hydroxyl-containing water-soluble polymers. This is, first of all, associated with the fact that the crosslinking activity of tetravalent titanium compounds is so high, that introduction of these compounds into an aqueous solution of hydroxyl-containing polymers, e.g. into a polyvinyl alchohol solution, with the polymer concentration over 3%, causes a rapid (taking as little as a few seconds) formation of the crosslinked polymer gel, after which the crosslinked polymer becomes unfit for use and transportation.
At present three ways of using tetravalent titanium for crosslinking aqueous solutions of polymers are known, yet none of them gives all the advantages stemming from the high crosslinking activity of tetravalent titanium.
The first way consists in reducing the crosslinking activity of tetravalent titanium compounds by combining them with other inorganic crosslinking agents, mainly with trivalent boron compounds. This method does not present any particular interest, since, being of palliative character, it does not eliminate the above-said disadvantages (formation of the polymer gel), but only defers the occurrence of the unfavourable pehnomena by reducing the crosslinking activity.
The second way, as known in the art, consists in the use of trivalent titanium compounds introduced into an aqueous solution of the polymer to be crosslinked together with an oxidant, which, after a period of time ranging from several minutes to a few dozens of minutes, oxidizes titanium to its tetravalent state the only state in which titanium crosslinking activity manifests itself. It should be noted, that the proportioning of the oxidant and the method of introducing it into the solution will govern the crosslinking process rate to no smaller extent than the concentration of titanium ions, i.e. instead of one crosslinking process parameter (titanium concentration) two parameters must be strictly controlled, so that carrying out the crosslinking becomes substantially more complicated. Moreover, any casual delay in the use of the prepared composition containing an aqueous solution of the polymer to be crosslinked, trivalent titanium and an oxidant will also lead at best, to the deterioration of the composition because of gel formation.
Both of the above-considered ways have one common substantial process disadvantage: after the polymer solution has been combined with the crosslinking agent the storage time of the prepared composition is determined only by kinetic factors, whereas chemical reactions leading to crosslinking commence immediately or/almost immediately after the components have veen combined and proceed with an equal rate until crosslinking is completed. Usually the storage time of the composition prepared for use amounts to a few minutes.
The third, most extensively used way consists in that the process of applying an aqueous solution of a polymer and the process of chemical crosslinking of the polymer by compounds of tetravalent titanium are carried out in two separate stages; this makes the entire process more complicated, increases its duration and requires additional costly equipment. This way is disclosed in U.S. Pat. No. 3,679,544, Cl. 162-157R. This Patent relates to producing waterresistant paper or nonwoven materials, and more particularly, to producing paper or nonwoven materials displaying resistance to water, especially to warm and hot water; in addition such paper possesses low moisture adsorption capacity and adequate resistance to washing; initial fibers of this paper or nonwovens are bonded together by means of a binder such as water-soluble polyvinyl alcohol fibers or polyvinyl alcohol resin with a dissolving point equal to 95.degree. C. Paper or nonwoven materials which consist completely or partially of polyvinyl alcohol are treated at about 40.degree. C. or lower with an aqueous solution of .alpha.-titanic acid containing at least 0.2 wt.% of metallic titanium, then washed and dried. In said U.S. Patent it has been revealed that water resistance, particularly to hot water, low moisture adsorption capacity and adequate washing resistance of polyvinyl alchohol paper and of nonwoven materials containing polyvinyl alcohol with dissolving point of 95.degree. C. or lower can be substantially improved by treating these materials at a temperature below 40.degree. C. with a solution produced by adding titanium tetrachloride to water at a temperature below 40.degree. C., or with a solution in a mineral acid of a residue produced by adding titanium tetrachloride to water, followed by the addition of ammonia. It has been further revealed that said treatment solution is unstable during storage, and that it can be stabilized by bringing it up to specific titanium: mineral acid concentration ratio or by producing a solution of titanium in a mineral acid having a concentration not below 40%, followed by addition of an alcohol.
In essence, said Patent relates to a method for producing a crosslinking agent based on a tetravalent titanium compound (.alpha.-titanic acid) to a method of stabilizing such compound, and to its application for crosslinking of polyvinyl alcohol.
The method of preparing the crosslinking agent, as disclosed in said U.S. Patent, resides in the following. Titanium tetrachloride is added to water at a temperature of 40.degree. C. and below. At a higher temperature a residue is formed, making the crosslinking agent unsuitable for treating paper or a nonwoven material according to the method described. Hydrolysis of titanium tetrachloride gives an aqueous solution containing approximately 15 wt.% titanium and 35-40% HCl, which for direct use is diluted in water to a concentration of titanium of 0.2-5.0 wt.%, usually to 0.5-2.0 wt.% to the weight of solution. With titanium concentration below 0.2 wt.% the desired water-resistance effect cannot be attained within the framework of the described method. With titanium concentration above 5 wt.% HCl concentration becomes equal to 14 wt.% and over, and a strongly acidic medium causes destruction of paper and nonwoven material.
The second embodiment of the method for preparing the same crosslinking agent which is a solution of .alpha.-titanic acid, resides in the following:
Titanium tetrachloride is added to water, then ammonia is introduced thereinto, the resultant precipitate of .alpha.-titanic acid is filtered and dissolved in mineral acids, for example, hydrochloric, sulfuric, nitric or orthophosphoric. This solution can be prepared with titanium concentration of about 15 wt.% and with the mineral acid concentration ranging from 35 to 40 wt.%. Hydrochloric acid solution is diluted prior to its use in the same manner as in the first embodiment. When using sulphuric acid, the solution is diluted to the titanium concentration of 0.2-5.0 wt.%, preferably 0.5-2.0 wt.%, and to the sulphuric acid concentration from 0.7 wt.% and over, preferably from 1.8 to 7.2 wt.%.
The treating solutions are relatively stable during storage, provided that titanium concentration amounts to 5% and over. When the solution is diluted to working concentrations of titanium (0.5 to 2.0 wt.%), a white precipitate of .alpha.-titanic acid is formed the content of this acid in the dissolved form diminishes and the solution loses its crosslinking capacity for polyvinyl alcohol.
In accordance with the cited U.S. Patent, stabilization of the crosslinking agent against precipitation of the .alpha.-titanic acid is accomplished by introducing a monohydric or polyhydric alcohol into the concentrated or working solution of the crosslinking agent.
As a stabilizing alcohol it is possible to use an aqueous solution of polyvinyl alcohol with a concentration below 3 wt.%, preferably 0.01-2.0 wt.% of the solution; titanium concentration in the solution to be stabilized should not in excess of 5 wt.%, preferably from 0.5 to 3.0 wt.%, and said polyvinyl alchohol solution and the crosslinking agent solution should be combined with sufficient care, since in case the polyvinyl alcohol solution, even diluted to the above-specified limits, is added rapidly, a hard-to-removed gel immediately forms on the surface of the equipment. If other alcohols are employed, they are introduced into solutions of the crosslinking agent in an amount of 40 to 95 wt.% so that after diluting these alcohols to the working concentrations of titanium (0.5-2.0 wt.%) the amount of the alcohol in the solution should be 1-10 wt.%, preferably 2-7% of the weight of solution.
In accordance with the cited U.S. Patent, the use of the crosslinking agent presupposes two technological operations: (a) introduction of polyvinyl alcohol in the form of fibers, as a powder or as an impregnating solution; (b) crosslinking the polyvinyl alcohol by impregnating a fabricated article (paper or a nonwoven material) with the above-described crosslinking agent solution. While in the case of introducing the polyvinyl alcohol binder the two-stage nature of the process is evident: impregnation with polyvinyl alchohol solution -- drying -- impregnation with the crosslinking solution -- drying, in the case of using polyvinyl alcohol fibers the double-stage nature of the process resides in that the first stage consists in shaping polyvinyl alcohol fibers on special equipment and involving problems of its own, while the second stage: impregnation with the crosslinking agent solution -- washing -- drying is carried out in the course of fabricating the moisture-resistant material.
Finally, when using a polyvinyl alcohol powder, the double-stage character of the process is even less evident. However, introduction of the polyvinyl alcohol powder into the rollers, refiners and the like equipment can be only considered as a version of the impregnation procedure, during which the binder introduction stage is combined with the stage of preparing the fibrous mass. This version has obvious disadvantages consisting in higher consumption of polyvinyl alcohol which is more expensive than cellulose, as compared with the version when polyvinyl alcohol is introduced into paper or into the nonwoven material only as an impregnating solution. The second essential disadvantage consists in low efficiency of polyvinyl alcohol as a binder when it is introduced as a powder at the material shaping stage.
The cited U.S. Patent teaches additional impregnation with a polyvinyl alcohol solution and subsequent drying to be used when producing paper from cellulose fibers bonded with polyvinyl alcohol in the disclosed manner.
Particular disadvantages of the method as disclosed in the cited U.S. Patent are as follows:
(a) the crosslinking agent; i.e. aqueous solution of .alpha.-titanic acid, is unstable under conditions of temperature increase and dilution to working concentrations of titanium. In the Patent under consideration lability is overcome by special introduction of a strong acid and or by adding a specific stabilizer, viz., a mono- or polyhydric alchohol, particularly, polyvinyl alcohol. These both ways as such constitute disadvantages of the crosslinking agent: the acidic medium promotes corrosion of equipment and necessitates the use of acid-resistant materials, while stabilization with alcohols is an additional operation in preparing the crosslinking agent and calls for additional consumption of chemicals (alcohols) which do not participate directly in the crosslinking reactions of the binder.
(b) The method of preparing the crosslinking agent requires control over the temperature of hydrolytic decomposition of titanium tetrachloride and presents certain difficulties in stabilizing the crosslinking agent with polyvinyl alcohols solution, since a slight increase in the rate of proportioning polyvinyl alcohols solution leads to the formation of a hard-to-remove gel on the equipment working surfaces, while the use of monohydric alcohols for stabilization requires the observance of safety rules prescribed for working with toxic organic solvents.
(c) When producing articles (paper or nonwoven materials) with the use of .alpha.-titanic acid solution as the crosslinking agent, the processes of applying the binder and of carrying out the crosslinking of the binder are conducted separately in two stages, or in the case of using polyvinyl alcohol binder in the form of a powder, when the stage of introducing the binder is combined with the stage of preparing a fibrous mass for shaping a web, higher consumption of polyvinyl alcohol is required, this being economically inexpedient and technically inefficient. There are still another two negative factors associated with the use of said crosslinking agent for impregnating paper or a nonwoven material. One of these disadvantages is the lengthy stage of impregnating with the crosslinking agent. In accordance with the description, the impregnation process lasts for at least 1 min. This signifies that if it is desirable to treat paper with said crosslinking agent directly on a papermaking machine operating even at such low rates as 60 to 100 m/min., it will be necessary to build-in an impregnation bath from an acid-resistant material with the length of the paper run in the impregnating solution of at least 60 to 100 m. Taking into account difficulites of paper threading in an aggressive (acidic) medium, it is apparent that attempts to combine the crosslinking technology under consideration in one unit with the existing papermaking process will be unrealistic.
The second disadvantage resides in the necessity of washing the paper or nonwoven material after treating thereof with the crosslinking agent, since the acid contained in the latter causes destruction of the paper and nonwoven material. The duration of washing is not indicated in the description, but the necessity in an additional operation comparable in duration with that of impregnation with the crosslinking agent is evident.